The invention relates to multilayer interference pigments with plateletlike titanium dioxide as substrate.
Multilayer pigments of low transparency are known. The metal oxide layers are prepared either in a wet process, by precipitating the metal oxide hydrates from a metal salt solution onto a carrier material, or by vapour deposition or sputtering under reduced pressure. In general, the vapour deposition processes are too complex and costly for mass production of pigments. Thus U.S. Pat. No. 4,434,010 describes a multilayer interference pigment consisting of a central layer of a reflecting material (aluminium) and alternating layers of two transparent, dielectric materials of high and low refractive index, for example titanium dioxide and silicon dioxide, either side of the central aluminium layer. This pigment is employed for the printing of securities.
JP H7-759 (Kokoku) describes a multilayer interference pigment with a metallic lustre. It consists of a substrate coated with alternating layers of titanium dioxide and silicon dioxide. The substrate is formed from flakes of aluminium, gold or silver or from platelets of mica and glass which are coated with metals. Accordingly, it is a typical metal-effect pigment. This pigment is of high opacity. For applications where a high level of transparency of the pigmented material is required, for example for agricultural films, the pigment is unsuitable. Furthermore, it has the disadvantage that the depth effect typical of interference pigments is not produced since, owing to the total reflection of the light at the metal layer which forms the core, a number of pigment particles are unable to enter into interaction. The interference effect therefore remains limited to the coats located on the metal layer.
Mica is the substrate employed most frequently for the production of interference pigments.
Mica pigments are used widely in the printing and coating industries, in cosmetics and in polymer processing. They are distinguished by interference colours and a high lustre. For the formation of extremely thin coats, however, mica pigments are not suitable, since the mica itself, as a substrate for the metal oxide coats of the pigment, has a thickness of from 200 to 1200 nm. A further disadvantage is that the thickness of the mica platelets in some cases varies markedly about a mean value. Moreover, mica is a naturally occurring mineral which is contaminated by foreign ions. Furthermore, technically highly complex and time-consuming processing steps are required including, in particular, grinding and classifying.
Pearl lustre pigments based on thick mica platelets and coated with metal oxides have, owing to the thickness of the edge, a marked scatter fraction, especially in the case of relatively fine particle-size distributions below 20 .mu.m.
As a substitute for mica it has been proposed to use thin glass flakes which are obtained by rolling of a glass melt with subsequent grinding. Indeed, interference pigments based on such materials exhibit colour effects superior to those of conventional, mica-based pigments. Disadvantageous, however, is that the glass flakes have a very large mean thickness of about 10-15 .mu.m and a very broad thickness distribution (typically between 4 and 20 .mu.m), whereas the thickness of interference pigments is typically not more than 3 .mu.m.
EP 0,384,596 describes a process in which hydrated alkali metal silicate is subjected at temperatures of 480-500.degree. C. to the action of an air jet, forming bubbles with thin walls; the bubbles are subsequently comminuted to give plateletlike alkali metal silicate substrates with a thickness of less than 3 .mu.m. However, the process is complex and the thickness distribution of the resulting platelets is relatively broad.
DE 11 36 042 describes a continuous belt method of preparing plateletlike or glitterlike oxides or oxide hydrates of metals of groups IV and V and of the iron group of the Periodic Table. In this method, a release layer comprising, for example, a silicone coating is first of all applied, if desired, to a continuous belt in order to facilitate the subsequent detachment of the metal oxide layer. Then a liquid film is applied which comprises a solution of a hydrolysable compound of the metal which is to be converted into the desired oxide, and the film is dried and subsequently detached using a vibration device. The coat thickness of the platelets obtained is given as being from 0.2 to 2 .mu.m, although no concrete examples of this are cited.
EP 0 240 952 and EP 0 236 952 propose a continuous belt method of preparing different plateletlike materials, including silicon dioxide, aluminium oxide and titanium dioxide. In this method, a thin liquid film of defined thickness of a precursor of the plateletlike material is applied, via a roller system, to a smooth belt; the film is dried and detached from the belt, forming plateletlike particles. The particles are subsequently, if desired, calcined, ground and classified.
The thickness of the platelets obtained in accordance with the method described in EP 0 240 952 is relatively well defined, since the film is applied very uniformly, for example to the continuous belt via a roller system. The layer thickness of the platelets is given in the examples as being from 0.3 to 3.0 .mu.m. According to Example 1, a first roller is wetted with the precursor used by immersing this roller partially into a stock container which is filled with the precursor. The film is transferred from this roller to a second, co-rotating roller which is in very close contact with the first roller. Finally, the film is rolled off from the second roller onto the continuous belt.
Disadvantages, however, are the use of very expensive precursor materials and, in particular, the increased requirements in terms of workplace safety which must be applied when organometallic compounds are used. The complete chemical conversion of the precursor into the desired coating material requires, in general, high heating of the film and of the belt material. In addition to the considerable thermal stress which this places on the belt material, the high energy consumption and the restriction on the process speed are also highly disadvantageous for the economy of the method.
WO 93/08 237 describes plateletlike pigments consisting of a plateletlike matrix comprising silicon dioxide, which may contain soluble or insoluble colourants and which is coated with one or more reflecting layers of metal oxides or metals. The plateletlike matrix is prepared by solidification and hydrolysis of water glass in a continuous belt.
DE 12 73 098 describes the preparation of a mother-of-pearl pigment by vapour deposition of ZnS, MgF.sub.2, ZnO, CaF.sub.2 and TiO.sub.2 films on to a continuous belt. This process however, like the process described in U.S. Pat. No. 4,879,140 in which plateletlike pigments with Si and SiO.sub.2 coats are obtained by plasma deposition from SiH.sub.4 and SiCl.sub.4, is associated with very high expenditure on apparatus.